1. Field of the Invention
This invention relates to the production of halogenated aromatic primary amines from the corresponding halogenated aromatic nitro compounds, the term "halogen" being used throughout this specification and claims to mean chlorine or bromine, so that the terms "halogenated" and "dehalogenation" mean "chlorinated or brominated" and "dechlorination or debromination" respectively.
2. Background of the Invention Prior Art
In the prior art process of producing halogenated aromatic primary amines, the corresponding halogenated nitro compounds are hydrogenated in the presence of Raney nickel catalyst. However, the desired hydrogenation reaction is accompanied by undesired dehalogenation, so that dehalogenated aromatic primary amines are formed as undesired by-products.
Therefore, one inevitable disadvantage of this dehalogenation had been that the yield and purity of the product is deteriorated. Indeed, the undesired by-products are made up primarily of dehalogenated aromatic primary amines. Fractional distillation is needed to eliminate them from halogenated aromatic primary amine, especially when the impurity content should be restricted.
Moreover, during the dehalogenation reaction hydrochloric acid is produced and the reaction mixture is thus made acidic, so that the surface of the Raney nickel catalyst is dissolved and the activity of the catalyst is extensively damaged, spoiling its usefulness.
Although many processes have been investigated to solve these problems, prior to our invention no practical solution was discovered. A platinum or palladium catalyst supported on carbon is mentioned in some patents. But such metals are very expensive.
A process for preparing halogenated aromatic amines by hydrogenation of the corresponding nitrocompounds using Raney Nickel catalyst in the presence of alkylamines, alkanolamines, basic heterocyclic compounds involving nitrogen atoms and alicyclic amines is described in Japanese provisional publication No. 49728/1973. However, the effectiveness of this previously described process is small as compared with our invention.
A process for preparing halogenated aromatic amines by hydrogenating the corresponding nitro compounds over a Raney nickel catalyst containing a thiocyanate is described in British Patent No. 1,191,610. But in this process, it takes a long time to hydrogenate the nitro compounds and activity of the catalyst is badly impaired by hydrogenation.
In Japanese patent provisional publication No. 49728/73 and U.S. Pat. No. 3,067,253, relatively low reaction temperatures, for example 22 .about. 40.degree.C or 55 .about. 60.degree.C, were employed to prevent dehalogenation. Some organic (Japanese patent provisional publication No. 49728/1973) and inorganic (U.S. Pat. No. 3,067,253) basic compounds mentioned in these patents were effective to some extent for hydrogenation at a lower temperature, but they are not useful at higher temperatures which are advantageous for industrial production. For example, in the hydrogenation of 2.5-dichloroaniline in the presence of Raney nickel and monoethanolamine, 3 percent of aniline and 2 percent of m- and 0-chloroaniline were detected by gas chromatography.